This invention relates to the production of methyl tert-butyl ether (MTBE); it concerns more particularly the isolation of said compound from the reaction product of methanol with a hydrocarbon cut containing isobutene.
Methyltert-butyl ether is known as a valuable industrial product; in particular its incorporation to fuels for controlled ignition engines provides gasolines of high octane number, particularly lead-free gasolines or gasolines of reduced lead content.
A difficulty in its preparation is due to the fact that the reaction of isobutene with methanol is a balanced reaction; it is consequently advantageous to proceed in the presence of an excess of one of the reactants, for example of methanol. However, when making use of a methanol excess or of an equimolecular mixture of the reactants, there is obtained, in practically all cases, a mixture containing methyl tert-butyl ether, methanol and hydrocarbons, for example the C.sub.4 hydrocarbons which were associated with the isobutene in the feed charge to the process as well as unconverted isobutene. The fractionation of these compounds by distillation is difficult to perform in view of the formation of azeotropes: the azeotrope of methanol with methyl tert-butyl ether and azeotropes of the C.sub.4 hydrocarbons with methanol.
Many proposals have been made for performing these fractionations but none of them is entirely satisfactory. Washing with water, for example, makes it possible to remove a substantial fraction of the methanol, but results in the presence of traces of water in the MTBE, which is a disadvantage for the incorporation of said ether to motor gasolines.
The distillation of the methanol / MTBE azeotrope is not advantageous in view, on the one hand, of the low methanol content of said azeotrope, even when operating under high pressures and, on the other hand, of the necessity to recycle said azeotrope to the reactor, which is unfavorable to the progress of the reaction.
The distillation of the methanol/MTBE/C.sub.4 hydrocarbons mixture, as issued from the zone of reaction of methanol with the C.sub.4 cut, is itself of low efficiency in view of the low methanol content of the methanol-C.sub.4 hydrocarbon azeotropes, even when proceeding under high pressure and at a high reflux rate.